Laying out mechanical, electrical and plumbing systems in new buildings under construction, or in existing buildings undergoing renovations or additions, is tedious, time consuming, and expensive. Typically, it has required a significant amount of labor to lay out construction points at a construction site on walls, ceilings and other surfaces, so that holes can be drilled and cuts made to permit the passage of pipes, conduits and the like, and to permit the installation of hangers, switches, fittings and other items. This layout process has required teams of workers that measure and mark the locations of these construction points, with much of the work being accomplished manually.
Manually measuring and accounting for all of these variables to locate construction points on walls and other vertical surfaces is difficult and time consuming. This process is subject to measurement errors and to accumulated errors which compound as successive measurements are made going from one intermediate point to the next. Further, building designs and requirements have become more complex, and construction schedules have become tighter, adding to the need to facilitate and simplify the layout process.
Robotic total stations are sometimes used for this purpose. For example, U.S. Pat. No. 8,031,332 B2 describes an iterative process using construction data with a robotic total station to direct a beam of laser light and establish a construction point.
A total station positioned at a known location directs a beam of laser light toward a target (e.g., an object or object point or construction point). By measuring the time of flight of the beam, the distance between the total station and the target is determined. By also measuring the direction of the beam from the total station to the target, i.e., the altitude and azimuth angles that define a vector from the total station to the target, the location of the target can be precisely determined.
Robotic or automated total stations are capable of locating an object point without being attended by an operator. Such total stations can be controlled to point in precisely determined directions. A total station can point to surfaces throughout a worksite and, by detecting the light reflected from those surfaces, determine the three-dimensional coordinates of the illuminated points throughout the worksite in relation to the position of the total station. If the coordinates and the orientation of the total station are known, the coordinates of the illuminated points are also known.
Robotic total stations are known to make distance and angle measurements, compute the location of the robotic total station relative to reference points, and then use the robotic total station's red distance-measurement laser as a pointer for layout of construction points. Trimble Navigation Limited provides such laser-pointer layout functionality in its field software products “Trimble MEP” and “Trimble Field Link for MEP” which are offered with its robotic total station models RTS555, RTS655, RTS633, RTS773 and other instruments. While these represent a significant advance over prior products, further improvements are desirable.
Using the red distance-measurement laser of a robotic total station as a pointer, e.g., for construction layout, has a number of drawbacks. First, the wavelength of the red laser makes it difficult for the human eye to see a spot projected on a target under daylight conditions. FIG. 1 shows the luminosity (human color perception) of light as a function of wavelength. For example, the luminosity of a typical red laser of 650 nm wavelength is about 0.1 in daylight and is effectively zero at night.
Second, the electronic-distance-measurement (EDM) optics are not well suited for use as a pointer. Visible-laser EDM optics have small aperture for the transmitted beam to separate the transmission path from the reception path and avoid coupling transmitted and received light. The EDM of a typical total station has coaxial paths for the transmitted beam and capture of light reflected from a target surface. The transmitted beam passes through a small center aperture, while the light reflected from the target returns through a much larger aperture having its central region blocked by the center aperture of the transmitted-beam path.
FIG. 2 schematically illustrates at 200 the optics of such a total station. A first optical path, which allows a user to view a target, is defied by an eyepiece 205 with reticle 210 and prism 215, a focusing lens 220 and a front lens 225. Focusing lens 220 is adjusted by a motorized focusing drive 230. The EDM laser beam 240 enters from the side and is deflected by a transmitter prism 245 to exit as a small-diameter beam through front lens 225 along optical path 250. Light 255 received along optical path 250 enters over the full aperture of front lens 225 and is shadowed at the center by the small-diameter transmitter prism 245. Received light is reflected by mirror 265 along a path 270 to an EDM detector (not shown). Mirror 275 reflects light along a path 280 to a target-tracking detector.
The transmitted laser beam travels in a straight direction, but has a divergence: its diameter increases with distance. The divergence is a function of the diameter of the laser beam at the lens that focuses or collimates the laser. That effect is called diffraction. The basic formula for the diffraction-related divergence angle is:
      sin    ⁢                  ⁢    α    =            1      ,              22        ·        λ              D  where α is the half divergence angle of the beam to its first diffraction minimum and A is the wavelength and D is the diameter of the limiting optics. Thus, the divergence of a laser is larger if the diameter that the laser uses at the optics is smaller. Typical divergence angles α are 0.1 to 0.2 mrad. This results in spot diameters of 10 mm to 20 mm at 50 m distance.
If the pointing beam is at a small angle (e.g., 20 deg) to the wall or ceiling, the laser spot width increases in one direction by a factor of three in that case. At the same time the visibility decreases by a factor of three. Thus a laser spot that is wide has very poor visibility, reducing precision of the layout task. Many operators compensate by using shorter distances and a larger angle to the wall/ceiling. Limiting the use cases to shorter distances and large angles needs more set-ups of the instrument per working area.
The divergence angle due to the small center aperture and fixed optics of the transmitted EDM laser beam means that the diameter of a spot projected on a surface increases significantly with distance from the robotic total station.
FIG. 3 schematically illustrates at 300 a layout scenario using a total station 305 to layout a point on a ceiling 310 using the EDM laser beam 315 to produce an unfocused laser pointer spot. The angle beta between the ceiling 310 and the beam 315 is shown at 325. A beam of width d_b shown at 320 produces a spot of width d_w shown at 330 according to the relationd—w=d—b/sin(beta)so that d_w≈3·d_b for beta=20 deg and d_w≈6·d_b for beta=9 deg.
FIG. 5A shows an example of a laser spot of size 5 having a brightness level 1. FIG. 5B shows an example of a laser spot of size 1 having a brightness level of 25. Laser power is limited by the laser class, e.g., a Class 2 laser has a 1 mW limit. A laser spot of 10 mm diameter has an area of 100 square mm, while a laser spot of 2 mm diameter has an area of 4 square mm. Thus the same laser produces a spot which is 25 times (100/4) brighter at 2 mm diameter than at 10 mm diameter, and is more visible even with lower laser power.
The user is thus tasked with identifying the projected red laser spot, whose color has low luminosity and whose diameter varies greatly with distance from the total station, and then tasked with visually estimating the center of the projected spot as the desired construction point.
A proposed solution to pointing with non-robotic theodolites is to remove the eyepiece of the theodolite telescope and replace it with a pointing laser. The pointing laser uses the optical path otherwise provided for the user to manually aim the telescope at a target. One example is the SwissTek Kern laser eyepiece having a green pointing laser whose dot size can be manually focused. A similar eyepiece-replacement solution, for industrial total stations, is the Leica DL2 Diode Laser Pointer having a red laser. A disadvantage of these manually-focused laser pointers is that the spot size at a given focus setting varies with distance from the instrument, so that to adjust the spot size requires manually resetting the laser focus for each measurement range during a layout project.
The Pentax R-300X series instruments have user-selectable lasers and prismless auto focus which focuses the EDM laser to get signal return at short range. A laser-pointer function turns the laser beam on continuously to become the aiming point for visual confirmation. The laser beam is designed not to be able to observe through the telescope. The user is instructed to visually align the laser beam to the target and mark the center. The user is instructed to confirm the horizontal and vertical alignment before measuring when performing accurate work like stake out when using the laser pointer function. The Pentax R-300X series instruments are not robotic, and thus not suitable for automated pointing. Thus, the user stands behind the instrument. In contrast, with a robotic instrument the user is at the target location to mark the construction point.
Another instrument, the Leica Disto 3D robotic pointer, lacks focusing of the laser pointer and thus has the issues of spot size and brightness discussed above.
Another instrument, the Trimble GX scanner, uses a focusable green laser for high-speed scanning of points, but is a different class of instrument unsuited to construction layout applications.
Improved apparatus and methods are desired.